This invention pertains generally to die cutting. More specifically this invention relates to anvil structure for use in conjunction with die cutting apparatus, the anvil structure defining a reaction surface for the cutting rules of die cutting structure.
Rotary die cutting pertains to the art of cutting a moving workpiece, e.g. a continuously moving web or a sheet of material, without interrupting the movements of the workpiece. In typical applications moving webs or sheets of material such as cardboard and corrugated paperboard, are passed between a cutting roller and an anvil roller. Cutting elements known as cutting rules are mounted on the cutting roller for rotation therewith. The anvil roller is provided with a cylindrical cover known as a cutting die blanket which fits around the surface of the anvil roller and effectively increases its diameter by twice the blanket thickness. The axes of rotation of the cutting roller and the anvil roller are parallel and displaced by an amount such that at their points of closest proximity the cutting rules penetrate the surface of the die blanket.
As the cutting rules penetrate the surface of the die blanket a resistance to the penetration is developed which, for purposes of this application, is called a reaction force. Adjustment of the relative positions of the axes of rotation of the cutting die roller and the anvil roller is made to provide a degree of penetration and therewith a degree of reaction force sufficient to insure complete cutting of the moving web or sheet of material.
In conventional die cutting apparatus the shafts of the cutting roller and the anvil roller are mechanically inter-engaged through a gearing pair. The gear of the anvil roller shaft may have one less tooth than the gear of the cutting roller shaft or, for the same purpose as discussed below, the diameters of the rollers may be slightly different. The purpose of such structure is to create a difference in the velocity of rotation of the cutting and anvil rollers so that the cutting rules will not continuously strike the same positions on the die blanket. Rather, the disparity in rotation permits the cutting rules to strike a different point on the surface of the blanket at each successive rotation thus prolonging the life of the cutting blanket.
As is well recognized in the art, however, there occurs situations wherein rotating apparatus not structured for differing degrees of rotation are adapted for use as die cutting apparatus. Typical of such situtations is the adaptation of printer-slotter apparatus to perform a die cutting function. Thus, it has been known to provide a printer-slotter apparatus with a cutting rule on the slotter knife shaft and a die blanket on the slotter head shaft such as to permit the formation of a manufacturers' joint concurrently with the slotting function of the apparatus during the manufacture of a box.
It is well recognized, however, that the slotter knife shaft and slotter head shaft of conventional printer-slotter apparatus are geared such as to cause the shafts to rotate at the same speed. Further, the plane of passage of the workpiece through the apparatus is positioned between the shaft axes such that the tangential velocity of the roller surfaces at the plane of the workpiece is equal. Modification of this structure to provide for die cutting results in continuous striking of the anvil blanket in the same position thus causing localized wear and shortened blanket life.
In order to accommodate for the localized wear on the die blanket, it is has been proposed to provide sliding anvil blankets. Known sliding anvil blankets have taken three basic structures, one structure had included a urethane die blanket slidably mounted on an anvil head, which head is rigidly secured to the anvil shaft. This structure has been disadvantageous because the die blankets, after a degree of usage, tend to deform in such a manner as to interfere with the capability of the blanket to slide on the anvil head. Once no longer able to slide, the wear again becomes localized and the life of the blanket is unreasonably shortened.
The other two basic known structures are similar. Each includes an anvil head rigidly mounted to an anvil shaft and a slip ring slidably mounted thereon, the slip ring having a coating of material such as urethane bonded to its surface. One of the basic structures slidably mounts the slip ring in a slot formed in the anvil head. The other provides for the slip ring to be secured to the anvil head by circumferentially mounting lock rings. In both cases the structure has been found to be disadvantageous because wear of the urethane coating requires the entire slip ring structure to be either discarded or returned to the manufacturer for resurfacing either alternative is economically undesirable.
Yet another disadvantage of known structures has been the proximity of the cutting rules to a solid metallic surface such as the anvil head. In such cases, where the cutting rule may come in contact with the surface of the anvil head, the cutting rule is often damaged.